Large quantities of cooking fluid are withdrawn from the digester in the current strainer constructions in continuous digesters. With production capacities of 4,000 tonnes of pulp per day, the digesters have a diameter greater than 8 meters, and this makes it difficult to withdraw the quantities that are required. The strainers are therefore heavily loaded, and it becomes important to maintain the full withdrawal capacity of the strainers during operation. Various types of strainer construction are used, such as strainers with strainer bars, slitted strainer plates or rods. These strainer constructions are mounted in one or several rows in the wall of the digester in order to achieve the withdrawal capacity required. The strainer surfaces in these strainer rows can be located around the complete circumference of the digester, or they can be arranged such that the row of strainers is built up from strainer surfaces with blind plates between the strainer surfaces. This is common in strainer sections that have several rows of strainers in which each strainer row is displaced between rows such that strainer surfaces are constructed also in the vertical direction with strainer surfaces and blind plates between the strainer surfaces, such that a strainer section with the pattern of a chessboard is obtained.
The chemical conditions at the withdrawal position are a further complication, where various types of precipitate can be activated in the withdrawal slots of the strainer or in the withdrawal compartment.
A further complication is the risk for the withdrawal of hemicellulose or other components that contain fibres, which may clog the withdrawal compartment behind the strainers.
It is normal that a digester plant has one planned maintenance stoppage per year, and the strainer sections are inspected and cleaned during this stoppage. It has proved to be the case at these stoppages that the withdrawal compartment in the strainer construction has been largely clogged with fibre precipitations or chemical deposits known as “scaling”.
A modified Kamyr digester is displayed in SE204097 in which alkali is added at the end of the cooking zone, and in which the cooking takes place in a countercurrent flow procedure. This process was developed by A. R. Sloman and was used in a digester installed in Tasmania that had a capacity of approximately 100 tonnes of pulp per day. An addition of black liquor to the withdrawal compartment took place in this system in a top separator with downward feed. This took place, however, with the aim of regulating the pressure in the digester, and the quantities added were small and depended on the pressure in the digester, being regulated by a valve 49.
A solution is displayed through U.S. Pat. No. 3,413,189 in our patent, in order to prevent the precipitation of lignin from the black liquor, in which a mixed cooking fluid is mixed with acidic pre-hydrolysate and black liquor. Extra alkali is added in this case at the centre of the digester through central pipes 55, which alkali is withdrawn to a large extent in the same position in the strainer section 15. Such large quantities of alkali are added here that the pH lies at a sufficiently high level, corresponding to a pH of 10 or 11.
Another technique applied to clean strainer sections has been to close down the withdrawal flow from the strainer such that the column of pulp that is led past the strainer section can exert a wearing effect on the slits of the strainer section, and in this manner provide abrasion of the clogging material on and in the slits. This cleaning, however, has an effect only on the outer parts of the strainer that face inwards towards the column of pulp.
The direction of the withdrawal flow in the header (the collection volume) has been changed in other techniques that are applied in order to make it possible to influence any sedimentation that has formed farthest away from the withdrawal taps. Other taps are opened in these systems in another header or at another position in this header, and the magnitude of the withdrawal flow is not affected.
A first aim of the invention is to avoid clogging of the withdrawal compartment of the strainer construction, by which measure the withdrawal capacity can be maintained for a longer period.
The digester can in this manner maintain a high withdrawal capacity during continuous operation, and it is not necessary to close down the digester or strainer section in order to clean the strainer construction.
A second aim is that measures taken to avoid clogging of the strainer construction are not to influence the cooking process of the chips in the column of pulp in the continuous digester. The solution can then be freely implemented in different continuous digesters that have problems with the clogging of digester strainers, without it being necessary to modify the cooking process.
The aims described above are achieved with a method to prevent the clogging of a strainer construction for a continuous digester.
Thus, in the method according to the invention sedimentation and the precipitation of deposits in the strainer compartment are avoided by the direct addition of an additive to the withdrawal compartment, which additive counteracts the precipitation process in the withdrawal compartment while the cooking fluid and the additive that has been added are withdrawn from the withdrawal compartment.
It is appropriate that the additive is added to the second subsidiary flow of the total flow that is withdrawn from the outlet tap, which second subsidiary flow is returned directly to the withdrawal compartment, and where the second subsidiary flow is less than or equal to the total flow. The additive can in this manner be caused to be mixed into the liquid volume in the withdrawal compartment either during recirculation or during withdrawal or during both recirculation and withdrawal of liquid from this volume of liquid.
Dissolved parts of sediment or chemicals can be bled from the strainer compartment in one advantageous embodiment that has the withdrawal of a first subsidiary flow of the total flow that is withdrawn from the outlet tap and that is led away from the strainer construction to a second location in the digestion plant or to the recovery process.
The rinsing effect of an activation of the second subsidiary flow is activated at least on regular occasions during the continuous operation of the digester when clogging in the withdrawal compartment is to be counteracted.
In one advantageous embodiment, the total flow is withdrawn from one end of the withdrawal compartment, preferably its lower part, and that the additive, preferably together with the second subsidiary flow of the total flow, is introduced into the second end of the withdrawal compartment, preferably its upper part. This is to ensure that the complete withdrawal compartment is to be rinsed through by the additive.
It is appropriate that the addition of the additive, preferably together with the second subsidiary flow, be regulated in a feedback manner that depends on a parameter that indicates the flow or the pH value of the first subsidiary flow or of the total flow.
Different additives can be relevant, depending on the type of risk of clogging that is present in any particular strainer construction.
If, for example, clogging can be caused by the precipitation of lignin as a result of a pH that is too low, an additive can be selected that influences the pH in the withdrawal space such that this is raised to a value of at least 10, preferably at least 11. Such an additive can be constituted by any one of white liquor, black liquor or alkaline filtrate from a subsequent bleaching plant, or constituted by mixtures of these.
If, for example, clogging can be caused by a temperature that is too low, an additive can be selected that is warmer that the liquid in the withdrawal compartment, and where the additive is partially or fully constituted by steam. If, for example, clogging can be caused by chemical precipitation onto the surfaces of the strainer construction, the additive can be principally constituted by a chemical that prevents the precipitation of chemical deposits. If, for example, the strainer construction is arranged at a position in the digestion plant at which the level of dissolved calcium ions is high, there is a risk for the precipitation of hard deposits (scaling) in the form of calcium carbonate. By the addition of carbon dioxide in gaseous or liquid form, the calcium carbonate can be caused to precipitate instead as crystals in the solution and can in this way accompany the liquid that is withdrawn from the strainer compartment.
Examples of the invention will be presented with the support of the embodiments described below.